The present invention relates, in general, to the field of electrical load monitoring systems and methods. More particularly, the present invention relates to a system and method of especial utility in the monitoring of the operating condition of various automotive electrical devices and for providing an indication of a device's operating condition to an operator located remotely therefrom utilizing a single electrical conductor.
Numerous applications exist for a system for monitoring the operating condition (open, shorted or operational) of an electrical device or load at a position remote therefrom, particularly in an automobile. In such a situation, there are obvious safety considerations in apprising the vehicle operator of the operational condition of the multitude of electrical devices on the vehicle, the condition of which can neither be directly observed nor repeatedly checked such that appropriate maintenance might be performed or a redundant device switched in.
To this end, various monitoring techniques have been hitherto proposed. The earliest of such systems involved the use of reed relays which would be closed by the electromagnetic force produced by the current drawn by an electrical load such as a lamp. In the event of a lamp failure, the reed relay would open a switch, resulting in an indication to the vehicle operator on the vehicle dash. A description of some typical reed relay systems is contained in a paper presented by F. J. Scharf to the Society of Automotive Engineers at the National Automobile Engineering Meeting in Detroit, Mich. on May 22-26, 1972 entitled "Automotive Lamp Outage Detection". Among the problems inherent in the use of reed relays are those noted in a paper presented by James McNamee and Brad Marshall at the same meeting entitled "Electronic Lamp Monitoring", such as lack of uniformity of the relays, tight tolerances on the proximity of the relays to the harness assemblies, and vibration in the automobile. Other obvious disadvantages include the expense inherent in the utilization of reed relays, the complexity of the wiring they necessitate, the fact that the relay cycles each time the load is switched on and the susceptibility of the system to voltage variations.
Another method of monitoring lamps has involved the use of phototransistors. Such light sensitive solid-state devices detect the actual presence of light at the respective lamps to be monitored. In such a system, the signals from each lamp are then brought to a common switch which can control the operation of an on-dash indicator. However, this system is also unsatisfactory because of the expense involved, the difficulty encountered in device placement in proximity to the monitored lamp, the inability to detect a single filament failure in a dual filament lamp and the need for calibration of the devices to various lamp types.
In light of the deficiencies inherent in the phototransistor approach, a fiber optic lamp monitoring system has also been proposed. In such a monitoring system, a plastic or glass fiber which transmits light by internal reflection is used which functions by physically conducting the light from a lamp to a jewel lens visible to the vehicle operator. Such a system results in a positive function indication, rather than an actual lamp outage detection. However, as with the phototransistor method, only a load which produces light can be monitored and only with respect to whether the device is either open or operational. No indication of a shorted condition can be provided.
Other systems include those employing a sensor such as a voltage comparator which compares the voltage drop of a vehicle's left side lamps to that of the right side lamps. This type of electronic circuit requires multiple subcircuits in order that all exterior lamps may be monitored. However, although many vehicle lamps may be monitored, the system is inherently expensive, susceptible to high frequency noise and requires a complex wiring harness to implement. An alternative electronic sensor approach involves the sensing of a load voltage drop which detects the difference between the drop for one lamp versus the electrical voltage drop for two lamps in the same circuit. However, once again the wiring harness required to implement such a system is relatively complex and results in a reduced voltage to the vehicle lamps which are to be monitored. Moreover, resistors or special vehicle wiring may also be required.
A further outage indicator system has been described in Automotive Electronics II published in February 1975 by the Society of Automotive Engineers at page 31. This system, which was proposed exclusively for stop lamps, involved the use of a high frequency oscillator connected between the stop lamp bulb and an inductor in series with the lamp switch. Should the lamp be burned out, a high frequency voltage is generated at the point of connection of the high frequency oscillator and the high frequency current flows through the inductor. This current flow provides an electrical input to light up an indicator lamp to alert the vehicle operator.
Overall, the aforementioned approaches are all relatively expensive to implement in terms of the numerous interconnecting wires they require between the monitored load and the indicator as well as in the way they function to sense current through the monitored load. The number of wires required in a vehicle wiring harness is a major consideration in system implementations and is the reason a multiplexed system approach is being adapted in the industry. Moreover, in using multiple-wire wiring harnesses, numerous connectors are necessitated thus introducing more failure prone points in the systems. Still further, with the systems above described, no differentiation is made between an operative or shorted condition of the monitored load and only an electrical open or operating condition may be sensed.